Improve the types for .data and .validated_data of the Serializers

[sbdchd]

class FooListSerializer(serializers.ListSerializer):
    ...

reveal_type(FooListSerializer(data).data) # list[Any]

class FooListSerializer(serializers.Serializer):
    ...

reveal_type(FooListSerializer(data, many=True).data) # list[Any]
reveal_type(FooListSerializer(data).data) # dict[str, Any]

need to investigate this more, what happens when passing many=True to a list serializer?

Also need to handle .validated_data as well

rel: https://github.com/encode/django-rest-framework/blob/98e56e0327596db352b35fa3b3dc8355dc9bd030/rest_framework/serializers.py#L117-L122

rel: python/mypy#8330 (comment)
rel: python/mypy#9482

[Viicos]

Related: typeddjango#260 (issue), typeddjango#315 (PR)